The present invention relates to a heat-shrinkable multi-layer film, and more particularly to heat-shrinkable multi-layer films which are excellent in sealing properties such as good sealability through contamination, stable sealing strength, wide sealing range, low-temperature sealability and hot tack property, optical properties such as clarity, impact strength, puncture resistance, low-temperature resistance, heat resistance, low extraction property, gas barrier properties, blocking resistance, slip property, and bag-making property. The heat-shrinkable multi-layer films according to the present invention are suitable for use as packaging materials for vacuum shrink packaging and modified atmosphere packaging (MAP) of, for example, processed meat products, fresh red meat (FRM) and cheeses.
Heat-shrinkable multi-layer films have heretofore been widely used in the form of vacuum shrink packages and MAP in fields of which good gas barrier property is required, such as processed meat products, fresh red meat (FRM) and cheeses. The heat-shrinkable multi-layer films are used as bag products subjected to sealing by a method such as heat, impulse or high frequency, or films subjected to packaging machines.
A representative basic structure of the conventional heat-shrinkable multi-layer films is composed of (1) a gas barrier resin such as polyvinylidene chloride (PVDC) resin or ethylene-vinyl alcohol copolymer (EVOH) resin, provided as a core layer, (2) a heat-sealing resin selected from the group consisting of polyolefin (PO) resins such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and very low density or ultra low density polyethylene (VLDPE or ULDPE), ionomer resins, ethylene-vinyl acetate (EVA) resins, etc., provided as an inner layer, and (3) a thermoplastic resin having excellent abuse resistance, slip property, package machinability, etc., provided as an outer layer. There has recently been proposed a multi-layer film provided, as a sealing resin layer, with a PO resin obtained by using metallocene catalyst (may also be referred to as single-site or Kaminsky catalyst).
However, the heat-shrinkable multi-layer films obtained by using such a known resin as describe above as a sealing resin layer have not achieved fully satisfactory performance as bag products under the circumstances. For example, the heat-shrinkable multi-layer films making use of LDPE in a sealing resin layer involve drawbacks that they have insufficient sealing strength and poor sealability through contamination because part (contamination) of the contents, which have adhered to sealing surfaces upon filling of the contents, form the cause of insufficient sealing. In the case where LLDPE is used as a sealing resin layer, the resulting heat-shrinkable multi-layer film has poor low-temperature sealability. In the case where EVA is used as a sealing resin layer, the resulting heat-shrinkable multi-layer film has insufficient sealing strength.
In the case where LLDPE or VLDPE polymerized using the conventional metallocene catalyst is used as a sealing resin layer, the resulting multi-layer film involves such problems that when the multi-layer film is extruded, motor torque in an extruder and pressure increase. Accordingly, productivity cannot be achieved. In addition, the resulting film with melt fracture (disordered irregularities) is insufficient in sealing properties such as sealing strength, fails to provide a beautifully finished seal line, impairs the appearance of a product packaged due to its poor clarity and moreover can not make the best use of the features of the polyethylene resins obtained by using the metallocene catalyst because its sealing strength becomes insufficient upon heat sterilization in the field of multi-layer films and casings having high shrinkage stress and heat shrink.
It is therefore an object of the present invention to provide a heat-shrinkable multi-layer film which is excellent in sealing properties such as low-temperature sealability, sealability through contamination and hot tack property, low extraction property, clarity, heat shrinkability, heat resistance, gas barrier property, etc.
Another object of the present invention is to provide a heat-shrinkable packaging material which is excellent in blocking resistance and slip property and is also excellent in high-speed bag-making property or high-speed filling property in a bag-making step or automatic packaging step.
A further object of the present invention is to provide a heat-shrinkable multi-layer film provided with a sealing resin layer having excellent blocking resistance, slip property, formability and processability compared with the ethylene-xcex1-olefin copolymers, VLDPE and EVA obtained by using the conventional Ziegler-Natta catalyst, or the polyethylene resins obtained by using the conventional metallocene catalyst.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. As a result, it has been found that when in a heat-shrinkable multi-layer film having at least a thermoplastic resin layer as the outermost layer, a gas barrier resin layer as a core layer and a sealing resin layer as the innermost layer, a layer formed of a resin material comprising a substantially linear ethylene-1-octene copolymer obtained by using a constrained geometry catalyst (CGC) is provided as the sealing resin layer, and an intermediate layer formed of a specific resin is provided at least between the outermost layer and the core layer, the above objects can be achieved. When a layer of the ethylene-1-octene copolymer is also provided as the outermost layer, a heat-shrinkable multi-layer film having good slip property can be obtained. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a heat-shrinkable multi-layer film comprising at least a thermoplastic resin layer as the outermost layer (A), a gas barrier resin layer as a core layer (B) and a sealing resin layer as the innermost layer (C), and optionally an adhesive layer between the individual layers, wherein
(1) the sealing resin layer of the innermost layer (C) is a layer formed of a resin material (b) comprising a linear ethylene-1-octene copolymer (a) obtained by using a constrained geometry catalyst and having an 1-octene content not lower than 1 wt. % but lower than 20 wt. % and a density higher than 0.885 g/cm3 but not higher than 0.960 g/cm3, and
(2) an intermediate layer (D1) formed of at least one resin (c) selected from the group consisting of polyamide resins, thermoplastic polyester resins and ethylene copolymer resins is provided between the outermost layer (A) and the core layer (B).
According to the present invention, there is also provided a heat-shrinkable multi-layer film wherein an intermediate layer (D2) formed of at least one resin (c) selected from the group consisting of polyamide resins, thermoplastic polyester resins and ethylene copolymer resins is further provided between the core layer (B) and the innermost layer (C).
The resin material (b) forming the sealing resin layer of the innermost layer (C) may preferably be a resin material comprising 10-100 wt. % of the ethylene-1-octene copolymer (a) and 0-90 wt. % of at least one polymer (d) selected from the group consisting of linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE) and ethylene-1-octene copolymer elastomers obtained by using a constrained geometry catalyst and having an 1-octene content of 18 wt. % or higher and a density of 0.885 g/cm3 or lower.
The thermoplastic resin forming the outermost layer (A) may preferably be at least one resin (e) selected from the group consisting of polyolefin resins such as linear low density polyethylene (LLDPE) and very low density polyethylene (VLDPE); thermoplastic polyester resins such as copolyester (Co-PET); polyamide resins such as aliphatic nylon and aromatic nylon; and the ethylene-1-octene copolymer (a) obtained by using a constrained geometry catalyst. In particular, when a layer of the ethylene-1-octene copolymer (a) is also provided as the outermost layer (A), a heat-shrinkable multi-layer film having excellent sealing properties, clarity and mechanical strength and far excellent blocking resistance and slip property is provided.